Microorganisms exist in complex multitrophic communities in nature. They rely on elaborate networks of chemical signaling to communicate with each other. The predator-prey relationship between nematophagous fungi and nematodes makes them an ideal system to study inter-species communication and co-evolution. We have established C. elegans and A. oligospora as a model system to study interactions between nematodes and nematophagous fungi. I found that A. oligospora and other closely-related species could sense the nematode pheromones, ascarosides, and induce trap morphogenesis. In addition, C. elegans are attracted to A. oligospora and a pair of olfactory neurons, AWCs, mediate this attraction. I propose to further elucidate the molecular mechanisms that govern this interaction. I will identify genes and signaling pathways required for ascaroside-sensing and trap morphogenesis in A. oligospora by studying its transcriptional regulation in response to nutrient starvation, ascarosides, and nematodes. I will also sequence the genomes of additional closely related fungal species to search for specific receptors that are only present in the species responding to ascarosides. Candidates identified by these approaches will be further assessed by other functional assays. To understand how AWCs mediate A. oligospora attraction, I will characterize the attractive fungal odors by GC-MS analysis and perform calcium imaging on AWCs. To identify candidate chemosensory GPCRs and downstream components that have roles in sensing fungal cues, I will profile the AWC transcriptomes by single-cell RNA-Seq. Mutants of the GPCR candidates will be assessed for chemotaxis towards A. oligospora odors and those with a chemotaxis defect will be further analyzed by calcium imaging. Mutants of other AWC-expressed genes that have a potential neuronal function will also be screened for defects in chemotaxis toward A. oligospora extract. This study will further our understanding of the molecular mechanisms that mediate interactions between nematodes and nematophagous fungi. In the future, these results will facilitate the development of new methods and strategies to control parasitic nematode infections.